In many cases, for measuring a weak magnetic field, a magnetic gradiometer (hereinafter also simply referred to as “gradiometer”) is formed using measurement data obtained by magnetometers for two or more magnetic field measurement regions. As an example of such gradiometer, a gradiometer using an optically pumped magnetometer is known. An optically pumped magnetometer applies a pump beam to a magnetic field measurement region with a group of gaseous atoms encapsulated therein to cause spin polarization and obtain a rotation of a polarization plane occurring when a probe beam for reading is made to pass through the region, as a signal according to a magnetic flux density of the region. Use of optically pumped magnetometers to obtain a difference between signals obtained in two respective magnetic field measurement regions when a probe beam has sequentially passed through the magnetic field measurement regions enables formation of a gradiometer. As an example of a high-sensitivity optically pumped magnetometer that can be used to form a gradiometer, U.S. Pat. No. 7,038,450 proposes an atomic magnetic sensor using a circularly-polarized beam as a pump beam and a linearly-polarized beam of light as a probe beam, for a cell in which alkali metal vapor is present.
However, for such gradiometer using optically pumped magnetometers, there have been no discussions ever before on an optimum geometric arrangement of a signal source and two magnetic field measurement regions and an optimum direction of a magnetic field to which the magnetometers respond in the two magnetic field measurement regions.